The present invention relates to a developing apparatus for use in an image forming apparatus. More specifically, it relates to a developing apparatus for developing a photoconductive drum of an image forming apparatus.
(1) Japanese Patent Publication No. 49586/1984 discloses a developing apparatus of an electrophotographic copying machine which comprises a housing adjacent to a photoconductive drum, an opposing developing roller and a following developing roller rotatably installed in the housing, and a supply mechanism for supplying developer to the developing rollers. In this developing apparatus, the distance between the photoconductive drum and the opposing developing roller is smaller than that between the photoconductive drum and the following developing roller.
Since the distance of the opposing developing roller to the photoconductive drum is smaller than that of the following developing roller, the conventional developing apparatus causes the phenomenon shown in FIG. 1. In FIG. 1, number 50 designates a photoconductive drum, number 51 designates an opposing developing roller, and number 52 designates a following developing roller. Arrows show the rotation directions of the rollers. Developer 53 is indicated by small dots.
As used in this specification and shown in FIGS. 1 and 3, the term "opposing developing roller" refers to a roller whose axis is turning in the same direction as the axis of the photoconductive drum and is positioned such that the surface of the opposing developing roller is turning opposite to the direction which the photoconductive drum surface is turning. The opposing developing roller is shown at reference number 51 in FIG. 1 and reference number 22 in FIG. 3. Likewise, in this specification, the term "following developing roller" refers to a roller whose axis is turning in the opposite direction as the axis of the photoconductive drum and is positioned such that the surface of the following developing roller is turning in a direction which follows the direction which the photoconductive drum surface is turning. The following developing roller is shown at reference number 52 in FIG. 1 and reference number 23 in FIG. 3.
In FIG. 1, the developer 53 drawn up first by the following developing roller 52 can pass through the gap between the rollers 51 and 52, and then about half that amount is transferred to the opposing developing roller 51. The two divided portions of the developer 53 are transferred toward the photoconductive drum 50 by the rotation of the rollers 51 and 52. Since the photoconductive drum 50 rotates in the direction opposite to the direction of developer 53 transferred by the opposing developing roller 51, there is a tendency to transfer the developer 53 from the opposing developer 51 toward the following developing roller 52 by the rotation of the photoconductive drum 50. Consequently, a portion of the developer 53 accumulates on the opposing developing roller 51, hanging like an icicle (see part indicated P in FIG. 1), and such portion of the developer 53 is reverted to the following roller 52. That causes an excess amount of the developer 53 to be supplied to the following developer 52, whereby the load on the following developing roller 52 is increased and copy quality is lowered. According to experiments performed by the inventors of the present invention, the phenomenon became more pronounced as the distance between the opposing developing roller 51 and the photoconductive drum 50 was made narrower than that between the following developing roller 52 and the photoconductive drum 50.
(2) United Kingdom Patent Application GB 2,141,048A discloses a developing apparatus of an electrophotographic copying machine which comprises a housing adjacent to a photoconductive drum, developing rollers rotatably installed in the housing, and a regulator plate disposed in the part of developer to be supplied to the developing rollers. In this developing apparatus, the gap between the regulator plate and the lower developing roller becomes filled with developer, because the gap the developer passes is narrow. The developer which has passed the gap is transferred forward the photoconductive drum by the developing rollers to develop the electrostatic latent image on the photoconductive drum.
In this conventional developing apparatus, developer is deposited between the regulator plate and the developing roller, wherein particles of the developer become magnetically adhered to each other due to magnetization by the developing roller. As a result, the developer becomes a hardened mass in the gap between the regulator plate and the developing roller, making rotation of the developing roller difficult. Consequently, increased torque on the developing roller is required, due to the heavier load on the developing roller.
Moreover, the gap between the regulator plate and the developing roller is not narrow enough whereby it may act as a measure for measuring the amount of the developer. The amount of developer that is drawn up by the developing roller is largely self-determined. That is, the entire amount of the developer drawn up by the developing roller by magnetic force can pass the gap between the developing roller and the regulator plate. As the developer is transported by the developing roller, it forms a magnetic brush on the surface of the developing roller corresponding to the magnetic force of the developing roller. Thus, the regulator plate of the developing apparatus cannot act to limit the length of the magnetic brush, whereby the load on and abrasion of the developing roller in the common type of developing apparatus are reduced, and wherein the regulator plate always controls the length of the magnetic brush by trimming the top portion of the magnetic brush.
In this conventional developing apparatus, as described above, the length of the magnetic brush is self-determined by means of the developing roller, wherein the length of the magnetic brush is stable and constant as long as the developing roller rotates at a constant speed. However, at the start of the developing process, or when the developing roller begins to rotate, the developing roller momentarily tends to carry a large amount of developer due to its slow initial speed of rotation. Therefore, in the conventional apparatus, a large excess amount of developer is supplied to the photoconductive drum, causing an increase of scattered toner and a sudden change in load.